Influence of age, chemical composition and rate of inclusion on the apparent metabolisable energy of fats fed to broiler chicks.

1. Vegetable oil (VO) and tallow (T) were evaluated at rates of 25, 50, 75, 100 and 125g/kg in a basal diet with broiler chicks aged 1.5, 3.5, 5.5 and 7.5 weeks of age using a total collection period of 72 h. 2. Apparent metabolisable energy, corrected to zero nitrogen retention (AMEN, MJ/kg) and apparent fat availability (AFA, g/kg) of the experimental diets were employed to estimate the dietary energy values of fats, by linear and quadratic regression. 3. AME of fats, determined from AFA, had smaller standard errors than those determined from AMEN. 4. There was a marked increase in the AME of fats from 1.5 to 3.5 weeks of age; thereafter, the improvement was less pronounced. 5. Rate of improvement in fat AME with age was greater with T than VO.